how to fix the creases after solidify and why it occurs?
This is z-fighting. It happens because of binary rounding errors when you have very thin/ overlapping geometry compared to your viewport or camera clipping range. Do not solidify your geometry before using a Shrinkwrap modifier, if you need an edge, use it after Shrinkwrap. Also you do not need all the geometry that is not visible, so you can have rim only: 



You have sides of the surface in vertex groups so I just selected the inner group and deleted it to fix the issue in the screenshot. You could have the Shrinkwrap modifier before solidify there if you needed it.